The invention relates to a process for the manufacture of spherical, surface-rich and pore-rich particles from activated alumina or activated alumina which may also contain silica and/or catalytically active metals and/or metal oxides. These particles are distinguished by good mechanical and thermal stability and are suitable as carriers for bulk or bed catalysts.
For application in stationary beds, particles of distinct spherical geometry and identical size are advantageous over tablets and/or extruded or granulated preformed particles. Spherical particles provide a dense and more uniform packing upon pouring. This, in turn, results in a symmetrical distribution of the interstitial spaces so that uniform flow conditions are guaranteed for the stationary catalyst bed. It is also established that tablets, extruded bodies, and granules show a greater tendency to break than spherical particles. The edges and peripheral regions of the former are likely to break away during processing and/or during application. This results in the formation of dust and powder-like material which tend to impair the flow through bulk catalysts and/or are potentially capable of inhibiting such flow completely.
Alumina particles of strictly spherical geometric shape and identical size may be manufactured according to known processes by application of the so-called oil-drop method. In these processes, aluminum oxide hydrosol is mixed with a base, for instance, hexamethylene tetramine, which is hydrolyzable at elevated temperature. This mixture showing an acid pH-value will be sprayed or allowed to drop into a liquid immissible with water and kept at elevated temperature, preferably a mineral oil. The specifically heavier sol droplets settle in the hot, precipitation-causing oil, they assume spherical geometry and they solidify to the gel under the influence of hydrolyzing effect of the base at the elevated temperature of the oil. The gel particles are still soft and not stable in aqueous solution. They are subjected to hardening by aging in hot oil. If necessary, they are subsequently treated under alkaline conditions before they are washed, dried, and calcined to convert them to activated alumina.
It is a serious drawback of this process, that the gel particles -- due to their high water content -- are subject to considerable shrinking upon the heat treatment. Tensions are generated as the consequence of this shrinking, which leads to the formation of cracks and the bursting of the particles. It has therefore been suggested in German Pat. No. 1,065,392 to add finely ground solid substances to the aluminum oxide hydrosol prior to the process of gel formation. Suitable solid substances are hydrate of alumina, activated alumina, silica gel, diatomaceous earth, Fuller's earth, or other catalytically active metal oxides. These solid substances may be added up to 90% by weight relative to the dry substance. These additives are not only intended for the prevention of the shrinking of the particles, and thus the formation of products which catalytically are of no use, rather, they are also added with the intention of providing an opportunity to modify the activity of the preformed particles as desired and to adapt the same to the given application as catalyst or catalyst carrier.
It has been observed according to the present invention, however, that despite the coprecipitation of aluminaor silica-containing fillers with gel-forming aluminum oxide hydrosols, a considerable proportion of the gel particles nevertheless become useless upon drying or calcination due to crack formation and/or bursting. The extent to which this proportion of useless particles is formed, is primarily a function of the particle size of the coprecipitated oxide or oxide hydrate as well as being a function of the quantity thereof. The addition of coarse-grained fillers results in a product, which, although the particles of which suffer only minor shrinking in the course of thermal treatment so that there is hardly any loss due to destruction of particles during production, exhibits mechanical stability of the particles inadequate for application as bulk catalysts or as carrier materials for catalysts. The finer the particle size of the coprecipitated component, the better is the resistance of the calcined particles to bursting and abrasion. However, the proportion of non-usable products increases strongly due to destruction during the heat treatment.
The above-described oil drop process according to the invention is according to a further embodiment of the invention also applicable to the direct manufacture of pellet-shaped catalysts without subsequent impregnation by the coprecipitation of catalytically active metals, metal salts or metal oxides.
It is already known that the activity of catalysts depends not only on the type of the metal compound applied but also on the process of incorporating the catalytically active metal into the carrier material and on the subsequent treatment of the product obtained. Most commonly the carriers are impregnated with solutions of the corresponding metal compound. Each such treatment is followed by the exposure to a high temperature process. Attempts to incorporate the active component in the form of solutions or as solids during the forming of the powder-like carrier material into extrudates, granulates or tablets result in the formation of catalysts having an activity which is considerably lower than that of the impregnated types.
It was therefore not to be expected that it would be possible in the process according to the invention to incorporate catalysts by the coprecipitation of compounds of catalytically active metals even during the forming of the carrier, and thus, to save a multitude of process steps.
German Pat. No. 1,065,392 describes the coprecipitation of finely ground alumina hydrate, activated alumina, silica gels, aluminum silicates, Fuller's earth, and other oxidic compounds suitable to function as carriers for catalysts, to which there may already have been added catalytically active components in metallic or oxidic form. See also German Auslegeschrift No. 2,027,878. Subsequent experiments have, however, shown that the coprecipitation of powder-like compounds of catalytically active metals and activated alumina or alumina hydrate in gel-forming aluminum oxide hydrosol results in the formation of spherical particles which do not display adequate catalytic activity.